There has been an increasing need for versatile visual displays for electronic products of many kinds. Light-emitting diodes ("LEDs") and liquid-crystal displays ("LCDs") have found many useful applications but have not been adequate in all cases. A visual display that is of relatively recent origin and that has shown much promise is the organic electroluminescent device. An electroluminescent device basically consists of an electroluminescent substance placed between a pair of electrodes. When an electric potential is applied across the electrodes, the electroluminescent substance emits visible light. Typically one of the electrodes is translucent, permitting the light to shine through.
FIG. 1 illustrates a typical electroluminescent device of the kind known in the art. A glass substrate 101 is coated with a translucent anode 103. A translucent hole transport layer 105 covers part of the anode and an electron transport layer 107 covers the hole transport layer, forming an interface 109 between the two layers. A cathode 111 covers the electron transport layer. In some devices the hole transport layer consists of two sublayers having slightly different composition, one sublayer forming a lower region 113 adjacent the anode and the other sublayer forming an upper region 115 adjacent the electron transport layer. The thicknesses of the anode, hole transport layer, electron transport layer and cathode are each of the order of 10-500 nanometers (100-5000 .ANG.ngstroms).
In operation, electric power from a voltage source 117 is applied to the anode and the cathode, biasing the anode positive with respect to the cathode. This causes regions of positive charge ("holes") to migrate through the hole transport layer from the anode toward the electron transport layer and electrons to migrate from the cathode through the electron transport layer toward the hole transport layer. The holes and electrons combine at the interface 115 between the two layers, emitting visible light. The light propagates out of the device through the hole transport layer, the anode and the substrate as indicated by an arrow 119.
It has been found that certain organic materials are particularly well suited for fabricating the hole and electron transport layers. An electroluminescent device fabricated of such materials is called an organic electroluminescent device. The anode of a typical organic electroluminescent device is made of indium tin oxide ("ITO"). Then the hole transport layer is formed by vapor deposition of N,N'-diphenyl-N-N'-bis(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine ("TPD"). Next, the electron transport layer is formed, also by vapor deposition, of aluminum tris-8-hydroxyquinoline (Alq.sub.3). Finally the cathode is formed by thermal evaporation of magnesium (Mg) and silver (Ag). Many different compounds and variations in structure have been used for the different layers and regions in organic electroluminescent devices. Examples of such devices and the specific compounds of which they are made are found in such references as U.S. Pat. No. 4,356,429 (Tang) issued Oct. 26, 1982; U.S. Pat. No. 4,539,507 (VanSlyke et al.) issued Sep. 3, 1985; U.S. Pat. No. 4,885,211 (Tang et al.) issued Dec. 5, 1989; U.S. Pat. No. 5,047,687 (VanSlyke) issued Sep. 10, 1991; and U.S. Pat. No. 5,059,862 (VanSlyke et al.) issued Oct. 22, 1991, U.S. Pat. 5,061,569 (VanSlyke et al.) issued Oct. 29, 1991, all of which are incorporated herein by this reference. See also Tang et al., Electroluminescence of Doped Organic Thin Films, JOURNAL OF APPLIED PHYSICS no. 65(9), May 1, 1989, pages 3610-3616.
Use of a conducting polymer as the anode in a flexible LED based on poly2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene! ("MEH-PPV") has been reported. This type of LED is fabricated by spin-casting MEH-PPV onto the anode to form the electroluminescent layers. The use in such a LED of a conducting film of polyaniline ("PANI"), either in place of or in addition to an ITO anode, is disclosed by Yang et al., Enhanced Performance of Polymer Light-Emitting Diodes Using High-Surface Area Polyaniline Network Electrodes, JOURNAL OF APPLIED PHYSICS, Jan. 15, 1995, pages 694-698; Cao et al., Solution-Cast Films of Polyaniline: Optical-Quality Transparent Electrodes, APPLIED PHYSICS LETTERS 60 (22), Jun. 1, 1992, pages 2711-2713; and Yang et al., Polyaniline as a Transparent Electrode for Polymer Light-Emitting Diodes: Lower Operating Voltage and Higher Efficiency, APPLIED PHYSICS LETTERS 64 (10), Mar. 7, 1994, pages 1245-1247. The reported advantages to using PANI in such LEDs include mechanical strength, reduced drive voltage, increased efficiency and adaptability to a flexible substrate.
A recurring problem with organic electroluminescent devices is a very short service life when continuously driven. Typically such a device has a service life of less than 30 hours before all luminance disappears. There have been many attempts to overcome this problem and to provide an organic electroluminescent device with a better service life. For example, it is proposed by Adachi et al. in Molecular Design of Hole Transport Materials for Obtaining High Durability in Organic Electroluminescent Diodes, APPLIED PHYSICS LETTERS no. 66 (20), May 15, 1995, pages 2679-2681, to use certain aromatic amines for the hole transport layer. Many aromatic tertiary amines that have been used to fabricate hole transport layers are disclosed in such references as U.S. Pat. No. 4,885,211 (Tang et al.) at column 14 and U.S. Pat. No. 5,059,862 (VanSlyke et al.) at column 9. Adachi et al. report service lives ranging from a few hours for devices having hole transport layers made of some such amines to as much as 500 hours for others. Adachi et al. suggest that devices having hole transport layers fabricated of those amines which provided the smallest energy barrier between the anode and the hole transport layer had the longest service lives.
Another proposal for increasing the service lives of organic electroluminescent devices by using amines for the hole transport layer is set forth in U.S. Pat. No. 5,374,489 (Imai et al.) issued Dec. 20, 1994. Imai et al. propose using 4,4',4"-tri(N-phenothiazinyl)triphenylamine ("TPTTA" for short) or 4,4',4"-tri(N-phenoxazinyl)triphenylamine ("TPOTA") for the hole transport layer. Using TPTTA for the hole transport layer (Example 1) resulted in a half-life of 385 hours, and using TPOTA (Example 2) yielded a half-life of 370 hours. By comparison, using TPD for the hole transport layer resulted in a half-life of only 131 hours. Better results were achieved by using two different amines, one for each of the two regions of the hole transport layer. In Example 3, TPTTA was used for the upper region of the hole transport layer adjacent the luminescent layer and a second amine characterized by a "star burst" molecule, that is a molecule having a stellar structure such as 4,4',4"-trisN-e-methylphenyl)-N-phenylamino!-triphenylamine ("MTDATA") was used for the lower region adjacent the anode. A half-life of 550 hours resulted. Example 4 was similar except that TPOTA was used for the upper region, resulting in a half-life of 530 hours.
In U.S. Pat. No. 5,306,572 (Ohashi et al.) issued Apr. 26, 1994, attention was concentrated on the interfaces between the various layers of the organic electroluminescent device. In one embodiment it was proposed to create an "interfacial" layer between the anode and the hole transport layer by treating one of the layers with a silane-coupling agent to reduce unevenness of the anode layer and improve adherence between the layers. The silane-coupling agent is a compound represented by the formula X-Si(OR).sub.3 where R is a hydrolyzable group and X is a functional group capable of reacting with the organic substance such as an amino, vinyl, epoxy or mercapto group or a halogen. Devices fabricated using various silane-coupling agents were claimed to have service lives of between 5,000 and 8,000 hours, compared to 10 hours for a device that omitted the silane-coupling agent. Service lives of up to 15,000 hours were claimed by forming a hydrogenated microcrystalline silicon film on the anode prior to using the silane-coupling agent. The repeatability of this work is uncertain, and the cost of implementing the technique in production may be higher than desired.
In U.S. patent application Ser. No. 08/508,020, (now U.S. Pat. No. 5,719,467) assigned to the one of the assignees of the present application, the inventors of the present application disclosed an organic electroluminescent device that uses a conducting form of a PANI film in its anodic structure to achieve greatly improved lifetimes. This organic electroluminescent device included a thin film of a conducting polymer such as polyaniline doped with camphor-sulfonic acid between the anode and the hole transport layer. In an alternative embodiment, the conducting polymer itself served as the anode. Service lifetimes of the order of 1000 hours were provided. However, in common with many other known organic electroluminescent devices, the light output and service life were substantially reduced by subjecting the electroluminescent device to temperatures in the range to which an electroluminescent device installed in an automobile dashboard, for example, could be subject when the car is parked in the sun.
From the foregoing it will be seen that there remains a need for an economical, reliable, durable, commercially-practical organic electroluminescent device whose service life and light output are not impaired by exposure to high temperatures.